There is a double tube (Green & Black plastic each about 4mm diameter)hanging into the footwell adjacent to the heater unit which is sucking air and makes a hissing noise.
Can someone tell me what this is or where it should be attached?

There are green and black vacuum lines that go to the vacuum motor on the front axle to slide the internal collar to engage the axles when in 4WD. Check the top of the axle on the passenger side for two open vacuum ports on the round case of the vacuum motor... When you move the transfer case into 4WD, does the "4WD" light come on in the indicator panel above the steering column?

My light does the same thing. Sometimes it takes a few seconds for the vacumn motor to engage the 4WD. As far as I know that is normal. But if the light does come on, then your vacumn system should be working fine.